Group transmissions with a multi-stage main transmission and at least one range group in drive connection downstream from the main transmission have long been known and are preferably used in heavy good vehicles. Often, there is also a multi-stage splitter group in drive connection upstream from the main transmission. By means of a usually two-stage range group with a gear interval above the total gear interval of the main transmission by approximately an average gear interval between two consecutive gear steps of the main transmission, the spread of the transmission as a whole is approximately doubled and the total number of gears available in the group transmission is also doubled. By means of a usually two-stage splitter group with a gear interval corresponding to approximately half of an average gear interval between two consecutive gear steps of the main transmission, the gear intervals of the main transmission are halved and the total number of gears available in the group transmission is again doubled. In that way, in combination with a three-step main transmission having three forward gears and one reverse gear, a 12-gear group transmission with a total of twelve forward gears and a maximum of four reverse gears is obtained. In combination with a four-step main transmission having four forward gears and one reverse gear, a 16-gear group transmission with a total of sixteen forward gears and a maximum of four reverse gears is obtained.
Compared with a single transmission with a comparable number of gears and similar gear gradation and spread, a group transmission has substantially more compact dimensions and lower weight. However, since many shifts in a group transmission require gearshifts in a number of partial transmissions and therefore take place with relatively complex sequences, most known group transmissions are shifted either in a partially automated or fully automated manner.
A typical design of a group transmission with a main transmission, a splitter group upstream from the main transmission and a range group downstream from the main transmission, is described in DE 44 22 900 A1 In this known transmission the main transmission is of countershaft design and has two countershafts and a main shaft, between which shiftable spur gear stages for three forward gears and one reverse gear are arranged. The splitter group is also of countershaft design, and comprises two spur gear stages also often referred to as shiftable input constants, which are arranged between the input shaft of the group transmission and the correspondingly extended countershafts of the main transmission. When the first input constant on the input side is engaged, the spur gear stage of the transmission-side, second input constant serves as a further shiftable spur gear stage of the main transmission. Furthermore, by means of a direct connection of the input shaft to the main shaft a direct forward gear can be engaged. The range group is of planetary design and comprises a simple planetary gearset with a sun gear, a planetary carrier supporting a number of planetary wheels and a ring gear, wherein the sun gear is connected to the main shaft in a rotationally fixed manner, the planetary carrier is connected rotationally fixed to the output shaft of the group transmission, and the ring gear can alternatively be locked relative to the housing to engage a gear ratio stage for a driving range with lower driving speeds, or else, it can be connected rotationally fixed to the output shaft to engage a gear ratio stage for a driving range with higher driving speeds.
Otherwise than in the group transmission known from DE 44 22 900 A1, the splitter group too can be of planetary design and the range group also of countershaft design. Thus, for example, from EP 0 618 382 B1 various designs of a group transmission are known, which have a countershaft-configured, multi-stage main transmission and an auxiliary transmission connected downstream from the main transmission. The auxiliary transmission comprises a two-stage splitter group and a two-stage range group. In a first version of the auxiliary transmission according to FIG. 2 of the said document, the splitter group is of countershaft configuration, while in contrast the range group, as in the group transmission known from DE 44 22 900 A1, is of planetary design. In contrast, in a second version of the auxiliary transmission according to FIG. 3 of the said document, the splitter group is of planetary configuration and the range group is of countershaft design.
Finally, from EP 1 825 168 B1 a group transmission is known, which comprises a main transmission of countershaft configuration, a two-stage splitter group of countershaft design connected upstream from the main transmission and a range group of planetary design connected downstream from the main transmission. In this case, in a manner known per se the range group is in the form of a simple planetary gearset with a sun gear, a planetary carrier supporting a number of planetary wheels and a ring gear, whose sun gear is connected rotationally fixed to the main shaft of the main transmission. Besides the engagement of two gear ratios for driving ranges with lower and higher speeds, in this range group provision is also made for engaging a reversing stage for a reverse driving range.
When the driving range for lower driving speeds is engaged, the ring gear is locked relative to the housing by a first shifting element and the planetary carrier is connected in a rotationally fixed manner to the output shaft of the group transmission by means of a second shifting element. When the reversing stage for the reverse driving range is engaged, the planetary carrier is locked relative to the housing by the first shifting element and the ring gear is connected rotationally fixed to the output shaft by the second shifting element. When the driving range for higher speed is engaged the first shifting element is in an open, neutral position (with no switching function) and the ring gear as well as the planetary carrier are connected by the second shifting element rotationally fixed to one another and to the output shaft.
By virtue of the reversing stage contained in the range group, it is advantageously possible to do without the arrangement of a shiftable reversing stage in the main group, i.e. the reversing stage present there as such can be omitted or replaced by a spur gear stage for an additional forward gear. However, to realize the shifting function of the range group, two shifting elements are needed, the first shifting element of which is in the form of a dual shifting element with two shift positions and a neutral position and the second of which is a triple shifting element with three shift positions among which is one shift position with triple coupling. Thus, the disadvantage of the range group design known from EP 1 825 168 B1 is the complicated structure in particular of the second shifting element and the need for two shift actuators for the simultaneous actuation of the two shifting elements.